Conventionally, there is known a rotary impact tool of the type including a drive shaft rotationally driven by an electric motor or a pneumatic motor and a hammer loosely fitted to the outer circumferential surface of the drive shaft. Cam grooves are formed on the outer circumferential surface of the drive shaft and on the inner circumferential surface of the hammer. Balls engage with the cam grooves of the drive shaft and the hammer so that the rotation of the drive shaft can be transferred to the hammer through the balls. As the hammer makes forward and rotating movement with respect to the drive shaft under the guidance of the cam grooves and the balls, it applies a rotary impact to an anvil provided with an output bit.
One example of conventional rotary impact tools is shown in FIG. 3. This rotary impact tool is disclosed in Japanese Patent Laid-open Application No. 2006-175553, wherein an output shaft 1 of a motor as a rotational power source is connected to a drive shaft 3 through a speed reduction mechanism 2 including a planetary gear mechanism.
A hammer 5 biased forwards by a spring 9 is loosely fitted to the outer circumferential surface of the drive shaft 3. Obliquely-extending V-shaped cam grooves 3a are formed on the outer circumferential surface of the drive shaft 3, while axially-extending straight cam grooves 5a are formed on the inner circumferential surface of the hammer 5. Balls 4 are arranged to engage with both the cam grooves 3a and the cam grooves 5a. Each of the cam grooves 3a has an obliquely-extending portion used in forward rotation and a reversely-extending portion used in reverse rotation. Rotation of the drive shaft 3 is transferred to the hammer 5 through the balls 4. The hammer 5 is provided with locking claws 6 protruding forwards.
An anvil 8 is rotatably supported on the front end portion of a gear case 7 by a bearing 70. The anvil 8 is provided at its front end with a chuck for holding an output bit and at its rear end with arm portions 8a rotationally engaging with the locking claws 6 of the hammer 5. The front end portion of the drive shaft 3 is rotatably supported within a bearing hole portion formed at the rear end of the anvil 8. Reference numeral 18 in FIG. 3 designates a housing.
When the work load is light, rotation of the drive shaft 3 is transferred to the anvil 8 through the hammer 5 by the engagement between the locking claws 6 of the hammer and the arm portions 8a of the anvil 8. If the work load becomes greater, the hammer 5 moves backwards against the spring 9 due to the angle of contact surfaces of the locking claws 6 and the arm portions 8a. At the time point when the locking claws 6 ride over the arm portions 8a, the hammer 5 is moved forwards by the biasing force of the spring 9. Due to the inclination of the cam grooves 3a, the hammer 5 rotates faster than the drive shaft 3 and strikes the anvil 8. As the anvil 8 is struck by the hammer 5 having the energy originating from the biasing force of the spring 9 and the rotational speed and inertial moment of the hammer 5, a large magnitude of torque is applied to the anvil 8. The drive shaft 3 continues to rotate while the hammer 5 reciprocates relative to the drive shaft 3 along the cam grooves 3a. Thus, the locking claws 6 of the hammer 5 ride over the arm portions 8a of the anvil 8. When the locking claws 6 strike the arm portions 8a next time, the hammer 5 strikes the anvil 8 in a state that it is rotated about 180° with respect to the anvil 8.
In this regard, the impact force of the hammer 5 against the anvil 8 becomes greater if the rotational velocity of the hammer 5 when striking the anvil 8 is higher. In other words, the rotational velocity of the hammer 5 can be found by the following equation from the kinetic energy conservation law:
spring energy of the spring 9 accumulated by backward movement of the hammer 5=total sum of the energy during rotation of the hammer 5=axial kinetic energy+rotational kinetic energy+spring energy. This can be represented by: KZmax2/2=MZv2/2+JZr2/2+KZ2/2, where K is a spring constant, Zmax is the backward movement distance of the hammer 5, M is the mass of the hammer 5, Zv is the axial velocity of the hammer 5, Zr is the rotational velocity of the hammer 5, Z is the bending deflection of the spring 9 and J is the inertial moment of the hammer 5.
The rotational striking impact applied to the anvil 8 by the hammer 5 is greatly affected by the second term, i.e., the rotational energy term, of the right-hand member in the above equation. Therefore, there is a need to increase the rotational velocity Zr at the striking time.
The rotational velocity Zr is given by the equation: Zr=Z·cos θ, where θ is the lead angle of the locus of the hammer 5. In order to increase the rotational velocity Zr, the lead angle θ of the cam grooves 5a is set small.
Conventionally, the rotational locus of the hammer 5 as seen in a development view is set to change linearly, which imposes the following constraints. The cam grooves 3a and the cam grooves 5a need to be formed at two points on the circumferential surfaces of the hammer 5 and the drive shaft 3. If the lead angle of each of the cam grooves 3a and the cam grooves 5a is made small within such an extent that the cam grooves 3a or the cam grooves 5a do not interfere with each other, it is difficult for the hammer 5 to have great enough axial displacement. This means that the energy accumulated in the spring 9 by the backward movement of the hammer 5 becomes small, consequently resulting in reduction in the rotational velocity of the hammer 5.